In a variety of water systems, bacteria or algae cause various problems. For example, in open-circulating cooling water systems, bacteria (e.g., zoogleal bacteria, filamentous bacteria, iron bacteria, sulfur bacteria, nitrifying bacteria, and sulfate-reducing bacteria), fungi (e.g., saprolegnia and penicillium), or algae (e.g., blue-green algae, green algae, and diatom) are grown, and slime or sludge is generated through deposition or accumulation of ooze-like contaminants formed through mixing of such microorganisms (main components) with, for example, inorganic substances (e.g., earth and sand) or dust. The thus-generated slime or sludge reduces thermal efficiency and water circulation, and causes local corrosion in apparatuses or piping. In some cases, bacteria grown in a water system cause damage directly to a human body, e.g., legionnaires' disease induced by Legionella dispersed from a cooling tower.
Also, in a paper mill water system, slime is generated through growth of, for example, various bacteria, fungi, or yeast, and the thus-generated slime causes defects (e.g., holes, spots, or blots) in a product, to thereby lower the quality thereof, or causes breakage of a paper sheet, to thereby reduce productivity.
Hitherto, such bacteria- or algae-caused problems in a water system have been prevented by adding a chlorine-containing oxidant like a hypochlorite salt to the water system. In general, when the residual chlorine concentration of water is 5 mg-Cl2/L or higher, growth of bacteria or algae in the water can be prevented. Since decomposition of such a chlorine-containing oxidant is promoted by UV rays, when a bactericidal/algicidal agent containing such a chlorine-containing oxidant as an active ingredient is charged into, for example, a plastic container and is stored or allowed to stand outside, the chlorine-containing oxidant is decomposed by UV rays. In the case of, for example, an open-circulating cooling water system to which such a bactericidal/algicidal agent has been added, difficulty is encountered in completely shielding the cooling water from light. Since piping or a heat exchanger of such a water system is made of, for example, copper or a copper alloy, decomposition of such a chlorine-containing oxidant is further promoted through elution of copper ions.
There is disclosed a technique for reducing decomposition of such a chlorine-containing oxidant; i.e., a method including adding, to a water system, a composition containing a hypochlorite salt, benzotriazole or tolyltriazole, and a sulfamate salt serving as a chlorine stabilizer, in which the effective chlorine component of the oxidant is stabilized by adjusting the pH of the composition to 13 or higher (see Patent Document 1).
When a sulfamate salt is added to a hypochlorite salt, a stabilized hypochlorite salt (combined chlorine), e.g., an N-monochlorosulfamate salt or an N,N-dichlorosulfamate salt is formed; i.e., the effective chlorine component of the hypochlorite salt is stabilized.
However, such a stabilized hypochlorite salt is considerably decomposed in a target water system containing a large amount of slime, or under strong UV irradiation or high-temperature conditions. Therefore, problems arise in that the concentration of the stabilized hypochlorite salt is difficult to maintain constant even under control of the amount of combined chlorine added by means of, for example, a batch timer or makeup water proportional control, and in that a large amount of a sulfamate salt as a stabilizer must be used.
In the case of, for example, a water system in which combined chlorine is consumed in a large amount, the amount of combined chlorine detected in the water system tends to be smaller than the amount of combined chlorine that has actually been added. When the amount of combined chlorine added is increased so as to maintain the effective chlorine detection level, running cost increases, and combined chlorine consumed in the water system remains therein a stabilizer in a large amount.
That is, when such a stabilized hypochlorite salt is decomposed in a water system, the resultant hypochlorite salt is consumed in the water system, but the resultant sulfamate salt remains therein. When the remaining sulfamate salt is discharged outside the water system, the sulfamate salt affects nitrogen regulation and COD regulation.
There is also proposed a composition containing a stabilized hypochlorite salt and a triazole compound (e.g., benzotriazole) (see Patent Document 2). However, when such a composition is used in a water system, the aforementioned sulfamate salt and a triazole compound (e.g., benzotriazole) are discharged outside the water system through blowing down, and the discharged salt and compound further affect nitrogen regulation and COD regulation.
Hitherto, a stabilized hypochlorite salt solution added to a water system has generally been prepared through in-line mixing of a hypochlorite salt solution and a sulfamate salt solution so that the hypochlorite salt and the sulfamate salt are present in equimole amounts.
This preparation requires a special control apparatus for in-line mixing of the two components so that they are present in equimole amounts.    Patent Document 1: Japanese Patent No. 3832399    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2006-206608